In the case of a known control device of this type (U.S. Pat. No. 5,311,952), the orientating device comprises a step switching mechanism which is driven by the drilling mud, the pressure of which is altered by reducing the pump pressure when it is necessary to actuate the step switching mechanism.
In the case of another known control device of the type mentioned at the outset (U.S. Pat. No. 5,215,151), the orientating device comprises a tubular differential piston which can be actuated by the drilling mud and which in the case of an axial displacement, is caused by a screw thread movement converting mechanism to produce a rotary movement and imparts a corresponding rotary movement to the second outer housing part which carries the directional drilling tool.
Another known control device (U.S. Pat. No. 5,339,913) describes two outer housing parts which are adapted to be telescopically extended and retracted in respect of each other, a screw thread movement converting transmission between the telescopic parts imparting a rotary movement to the outer housing part which carries the directional drilling tool.
Common to the prior art control devices is the fact that the measuring and controlling device is disposed underneath the orientating device.
If in the case of the previously known control devices there is a fresh orientation of the directional drilling tool, then essential operating parameters such as the pressure of the drilling mud or the weight loading of the drill bit which goes hand in hand with a variation in the twist of the coiled tubes will vary and upon resumption of drilling operation, there wilt be a furs variation. This means that in practice the process of orientation becomes time-consuming since it is subject to various interference variables which render prolonged testing inevitable.